Nitrate wastes represent an environmental problem. Nitrate-based solvents are used in weapons research and production. Mining, chemical, farming and other industries are also major generators of nitrate wastes. Such wastes contaminate natural waters if released into the environment. The strong oxidizing power of nitrates causes excessive biological activity, growth of the vegetation, and precipitation of organic residue to the bottoms of streams, rivers, lakes, and oceans. As a result of precipitation of organic residue from nitrate waste streams, water reservoirs become shallow and eventually are converted to marshes and decay.
There have been developed several methods of treating nitrates in industrial and agricultural waste streams to convert the nitrates to less environmentally hazardous substances, but no simple, economical process for destruction or separation of nitrates from aqueous solutions is known. Generally nitrate wastes are treated by concentration in reverse osmosis, electrodialysis, evaporation, and ion exchange processes. Although the nitrates are at much reduced volume and increased concentration, the products of these processes are still nitrate wastes.
Evaporation is used to convert the nitrates into solid nitrate salts which can be thermally destroyed. Usually thermal treatments result in undesirable nitrogen oxides. Both evaporation and thermal destruction are very energy intensive processes.
Although nitrates can be reduced to nitrogen gas and ammonia by biological destruction, this process is slow, difficult to control and results in an organic residue.
Nitrates have been treated with supercritical water or wet air oxidation methods. However, these methods require extreme temperatures and pressures. The equipment is not easy to operate particularly when dealing with waste streams having inconsistent contents or concentrations. Products of nitrate reduction with supercritical fluids may contain undesirable NO.sub.x components.
There is still a need for easily controlled methods of converting nitrate wastes at moderate temperatures and pressures to more environmentally benign substances.
Therefore, it is an object of this invention to provide a method of treating nitrate waste streams to obtain more environmentally benign substances.
It is another object of this invention to provide a method of reduction of nitrates to nitrogen gas.
It is a further object of this invention to provide a method of reduction of nitrates to carbon dioxide and nitrogen gas in which any metals used are recovered and recycled.
It is yet another object of this invention to provide a simple continuous process by which waste streams containing nitrates can be treated to reduce the nitrates to environmentally benign substances.
Additional objects, advantages and novel features of the invention will be set forth in part in the description which follows, and in part will become apparent to those skilled in the art upon examination of the following or may be learned by practice of the invention. The objects and advantages of the invention may be realized and attained by means of the instrumentalities and combinations particularly pointed out in the appended claims. The claims appended hereto are intended to cover all changes and modifications within the spirit and scope thereof.